Merry Christmas. Because I celebrate Christmas, and thats how i greet you during this time of year. if you want to greet me with happy Hanukkah, Kwanza, or just Happy Holidays thats fine. but you cant take Christmas away from me, no one ever can.
It's Merry Christmas. Even the small number of people that are a different religion (which ironically for most of my friends that aren't "a different religion" actually religion plays no part in their Christmas anyway) wish me Merry Christmas. They still get time off work, spend time with their families and buy each other presents anyway. Why are we off work? Because it's Christmas.
But then, in my country "holiday" means getting on a plane to somewhere hot, not just being off work. We use "holiday" for what you use "vacation" and we use "vacation"... never. So anyone that said "happy holidays" here would be looked at like they were retarded.